1. Field of the Invention
The present invention relates to a rare-earth-iron-boron permanent magnet.
2. Description of the Related Art
Recently, permanent magnets containing rare earth, Fe, and, B as the basic components have been closely studied, and the results of these studies have been published in patent documents and the like.
Japanese Unexamined Patent Publication No. 57-141901 discloses a method for producing a permanent magnet powder wherein the composition of a transition group metal (T), metalloid metal (M), and a lanthanoid element (R) is glassified, and the obtained amorphous composition is then crystallized and a coercive force is generated by heat treatment. According to this publication, T is one or more elements selected from Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zr, Nb, Mo, Hf, Ta, and W; M is one or more elements selected from B, Si, P, and C; and R is one or more elements selected from Y and lanthanoid elements. This publication claims a permanent magnet powder expressed by the formula (T.sub.1-x M.sub.x).sub.z R.sub.1-z, wherein 0.ltoreq.x.ltoreq.0.35 and 0.35.ltoreq.z.ltoreq.0.90.
Japanese Unexamined Patent Publication No. 58-123853 proposes a La- and Pr-containing material having the composition (Fe.sub.x B.sub.1-x).sub.y -(La.sub.z Pr.sub.w R.sub.1-z-w).sub.1-y, in which R is one or more rare earth elements except for La and Pr, x=0.75.about.0.85, y=0.85.about.0.95, z=0.40.about.0.75, w=0.25.about.0.60, and z+w.ltoreq.1.0. According to this publication, the kinds and proportion of rare-earth elements are adjusted to provide the above composition (La.sub.z Pr.sub.w R.sub.1-z-w) so as to attain an appropriate enhancement of the coercive force at the annealing and crystallizing of the rare earth-iron-boron alloy. The coercive force is enhanced at approximately 3 kOe.
Japanese Unexamined Patent Publication No. 59-46008 proposes a magnetically anisotropic sintered body consisting of from 8 to 30 atomic % of R (at least one of the rare earth elements), from 2 to 28 atomic % of B, and Fe in balance. The invention of this publication aims at producing a permanent magnet having a desired shape by the sintering method, since the method of rapid cooling the melt brings about certain limitations in the magnet shape. The above publication discloses, as R, Nd alone, Pr alone, a combination of Nd and Pr, a combination of Nd and Ce, a combination of Sm and Pr, Tb alone, Dy alone, Ho alone, and a combination of Er and Tb.
The above prior arts disclose that excellent magnetic properties are obtained for the rare earth-iron-boron magnet, in which the rare earth element is Nd or Pr. In addition, La and Ce are set forth in the claim in the unexamined patent publications as the rare earth elements, but the highest content of La and Ce are limited so as not to incur a reduction in the magnetic properties. There is a substantial absence of disclosure directed to the rare earth-iron-boron permanent magnet, the rare earth components of which are mainly composed of La and Ce. This is further explained with reference to FIG. 1.
Referring to FIG. 1, Pr and Nd as the rare earth components of the rare earth-iron-boron permanent magnet exhibit the best magnetic properties. When La or Ce is used as the rare earth component, the alloy consisting of La or Ce, Fe, and B cannot exhibit the same magnetic properties as the permanent magnet. FIG. 1 teaches that the replacement of Nd, and Pr with La or Ce causes a reduction in the magnetic properties required for the permanent magnet. Based on the teaching of FIG. 1, it can be said that the prior arts explained above teach R-Fe-B alloy which can exhibit the magnetic properties required for the permanent magnet only at a slight replacement of Nd and Pr with La or Ce but not an alloy wherein the rare earth elements are composed mainly or totally of La or Ce.
A recent prominent advancement of the rare earth-iron-boron permanent magnet is disclosed in the publication "DIDYMIUM-Fe-B SINTERED PERMANENT MAGNETS" at MMM on October 1984, which attained a coercive force (iHc) of 10.2 kG and a maximum energy product ((BH) max) of 40MGOe by a magnet consisting of 32.5.about.34.5% of R, 1.about.1.6% of B, and balance of iron, wherein R is (Nd -10% Pr), 5% Ce-didymium, or 40% Ce-didymium. In this permanent magnet, the main rare earth component is also Nd.
Japanese Unexamined Patent Publication No. 60-100402 discloses a method in which melt containing Fe, B, and Nd and/or Pr is rapidly cooled to form amorphous or finely crystalline, solid material, and further, it is subjected to a high-temperature treatment by hot-pressing to form a plastically deformed body having a microstructure formed by fine particles, followed by cooling.
The time duration of the high-temperature treatment and the cooling speed are adjusted to induce a magnetic anisotropy in the resultant permanent magnet body.
One of the drawbacks of the permanent magnet, the main components of which are rare earth elements Fe, and B, is that Nd, or Pr must be the main components of the rare earth elements to attain excellent magnetic properties, and hence the permanent magnet becomes expensive. The permanent magnet containing dydimium is attractive, since the dydimium is inexpensive, and further, the permanent magnet can exhibit magnetic properties comparable to magnets containing Nd and Pr.
If La or Ce can be contained in the rare earth-iron-boron magnet as a main component(s) of the rare earth components, a drastic cost reduction of such a magnet becomes possible, since La and Ce are available in a greater amount than the other rare earth elements and hence are inexpensive. Nevertheless, La and Ce are detrimental to the magnetic properties, as is understood from FIG. 1. The ferromagnetic crystal of the rare earth-iron-boron magnet is an R.sub.2 Fe.sub.14 B compound which becomes unstable or is not at all formed when R is La. When R is Ce although R(Ce).sub.2 Fe.sub.14 B is formed, the coercive force of this compound becomes low.
As described above, there is a substantial absence of any disclosure in the prior art for replacing Nd, Pr, and the like with a large quantity of La or Ce.
The plastic working method disclosed in Japanese Unexamined Patent Publication No. 60-100,402, i.e., the hot-working method, involves a problem in that: an appropriate temperature for the plastic working is from 700.degree. C. to 850.degree. C. and thus relatively high; the pressure is from 1 to 3 ton/cm.sup.2 and relatively high; and, an appropriate pressing time is approximately 5 minutes and thus relatively short. According to this publication, during plastic working of the microstructure material the magnetic anisotropy is induced and the magnetic properties are therefore improved. To improve the magnetic properties, it is crucial to control the plastic working in terms of temperature, pressure, and time in such a manner as mentioned above. Such control is complicated. If the control is unsatisfactory, not only are the desired magnetic properties unobtainable, but also the shape and dimension of the products is restricted, so that products appropriate for various uses cannot be obtained, and this is a drawback in industrial application. If an appropriate temperature for the plastic working becomes low, and if the pressure for the plastic working becomes low, the plastic working method can be broadly applied for the production of various shapes, for example, an extremely thin magnet.
The anisotropic magnet having a radial direction of anisotropy is well known in the field of plastic magnets. The magnetic powder generally used for the radial anisotropic permanent magnet is Sm-Co powder. The rare earth-iron-boron magnet has a drawback that, when pulverized, the coercive force is decreased. Because of this, it has been heretofore difficult to produce a radial anisotropic permanent magnet using the rare earth-iron-boron powder.